The Bridgman method and the gradient freeze method have been known as methods for growing a single crystal.
The Bridgman method which is one for growing a single crystal, comprises the steps of; putting a crucible filled with a polycrystalline raw material in a crystal growth furnace, heating the crucible by a heater to melt the material, and thereafter moving the crucible toward a lower temperature side in a temperature gradient of 10-50 .degree. C./cm which is formed in the furnace, to precipitate a solute which cannot keep dissolving in the solvent, on a lower temperature portion of the crucible. For example, Japanese Patent Application Publication (Examined) No. Tokuko-hei 7-51471 discloses a technique for obtaining a single crystal having a good stoichiometry and few defects, in which a reaction tube which connects a crystal growth chamber enclosing a container (crucible) containing a raw material therein to a storage chamber enclosing a host atom for controlling the vapor pressure thereof is used, and growth of the single crystal is carried out while controlling the vapor pressure of the host atom by controlling the temperature of the storage chamber enclosing the host atom.
The gradient freeze method which is one for growing a single crystal, comprises the steps of; heating a crucible filled with a polycrystalline raw material by a heater to melt the material in a crystal growth furnace, and thereafter changing the temperature profile in the crystal growth furnace by controlling the power supplied to the heater while retaining the crucible at a predetermined position, to precipitate a solute on a lower temperature portion of the crucible.
The present inventor proposed an improved technique of the Bridgman method or the gradient freeze method, which is one (hereinafter, it may be simply referred to a solvent method) for growing a compound semiconductor single crystal having a higher melting point, e.g., ZnSe, ZnTe or the like, than the softening point of quartz glass, comprises the steps of; putting a raw material and a solvent in a crucible, heating the crucible to make a solution by dissolving the material in the solvent, and moving the crucible toward a lower temperature side in a temperature gradient which is formed in a crystal growth furnace, or changing the temperature profile in the furnace, to precipitate a solute from the solution. This solvent method has the advantage that it is possible to easily obtain a compound semiconductor single crystal having a high melting point and a high dissociation pressure at low cost.
However, even if either of the Bridgman method and the gradient freeze method is adopted for the solvent method, there is a problem of occurrence of cracks or defects in the grown single crystal caused by thermal stress due to the difference between the coefficients of thermal expansion of the remaining solvent in the crucible and of the grown crystal, in a cooling step after the single crystal growth. The coefficient of thermal expansion of Te is 14.times.10.sup.-6 /K, and the coefficient of thermal expansion of ZnTe is about twice that of Te. Therefore, when a ZnTe single crystal is grown in a Te solvent, cracks occur or defects such as dislocation or the like are sometimes introduced into the grown single crystal, by the remaining Te solvent in the crucible even if cooling after the crystal growth was sufficiently gradually carried out.
The Journal of Crystal Growth, 72, p.97-101 (1985); the Journal of Crystal Growth, 59, p.135-142 (1982); and the REVUE DE PHYSIQUE APPLIQUEE, 12, P.151-154 (1977) reported solvent evaporation processes for precipitating CdTe solute by evaporating Cd solvent from a solution which comprises the Cd solvent and the CdTe solute, in order to grow a CdTe single crystal. In the process, a crucible 20 containing suitable amounts of Cd and CdTe therein is sealed in an ampoule 21, as shown in FIG. 3; and then only the temperature of a reservoir portion 26 is gradually shifted to the temperature profile below the point "A", which is shown by a dotted line, in a graph in the right side of FIG. 3, for showing the temperature profile, without changing the temperature of a crystal growth region 25, by adjusting the outputs of heaters 23 and 24; and thereby a single crystal 28 is grown while evaporating the solvent in the solution 27. The evaporated and thereafter condensed solvent 29 is accumulated in the reservoir portion 26. However, the solvent evaporation process has the disadvantages in that the process has no effect of gettering impurities by the solvent during the crystal growth, it is difficult to strictly control the evaporation rate of the solvent, and the process requires a complicated structure for communicating the crystal growth region 25 in the ampoule 21 with the reservoir portion 26 through an orifice 30.
The present invention has been developed so as to solve the above problems. An object of the present invention is to prevent introduction of cracks or defects into the grown crystal and to obtain a compound semiconductor single crystal having a good crystallinity and no cracks.